You can't over feed if you are exclusively breastfeeding. You can offer as frequently as you please, and it is not overfeeding. If your baby chooses to nurse, then he needs to nurse. Breastfed babies tend to pack on lots of weight early on, then slim down when they get more mobile. My baby was super fat as an infant - triple layer rolls fat - but he slimmed down significantly around his first birthday and is fairly proportional now.
If walking your baby in the Ergo gets him to sleep, then that is what you should do. I've done exactly the same thing on numerous occasions.
K. Sophia - Mama to my little lactivore, the amazing Mr. X (11/10).